CORD-3183 Fix synchronization of instances that use keys

Change-Id: I8910c560e762e011341595212b26d81c23734313
diff --git a/xos/synchronizer/steps/sync_openstackserviceinstance.py b/xos/synchronizer/steps/sync_openstackserviceinstance.py
index 5e0b058..04f71b9 100644
--- a/xos/synchronizer/steps/sync_openstackserviceinstance.py
+++ b/xos/synchronizer/steps/sync_openstackserviceinstance.py
@@ -38,10 +38,10 @@
         pubkeys=[]
 
         if instance.slice.creator and instance.slice.creator.public_key:
-            pubkeys.add(instance.slice.creator.public_key)
+            pubkeys.append(instance.slice.creator.public_key)
 
         if instance.slice.service and instance.slice.service.public_key:
-            pubkeys.add(instance.slice.service.public_key)
+            pubkeys.append(instance.slice.service.public_key)
 
         userdata = '#cloud-config\n\n'
 #        userdata += 'opencloud:\n   slicename: "%s"\n   hostname: "%s"\n   restapi_hostname: "%s"\n   restapi_port: "%s"\n' % (